Crop Profile for Onions in California - Regional IPM Centers

Prepared: October, 1999
Crop Profile for Onions in California
General Production Information
● California is the number one producer of onions in the US, producing 26% of the nation’s onions.
(1)
● In 1996, California farmers planted 26,600 acres of dehydrator onions, and 21,000 acres of fresh
market bulb onions resulting in yields averaging 19 and 20.2 tons per acre, respectively. (13,14)
● The total value of California’s onion crops in 1995, 1996 and 1997 were $135 million, $128
million and $144 million, respectively. (8)
Production Regions
Onions are grown throughout California (Appendix 1). Dehydrator onions are produced in Imperial,
Kern, and Fresno Counties and also in the Tulelake Basin (Siskiyou and Modoc Counties). Fresh market
bulb onions are produced in five main areas: Low Desert (Imperial and Riverside Counties), San Joaquin
Valley (Kern, Fresno and San Joaquin Counties), High Desert (eastern Los Angeles County), Central
Coast (San Benito, Ventura, Santa Clara and Monterey Counties), and the Northern Mountain Valleys
(Siskiyou, Modoc, Shasta and Lassen Counties). (13,14)
Area Plant Harvest
Low Desert October – November May – June
San Joaquin Valley October – February June – October
Northern Mountain Valleys April September – October
High Desert March – April August – September
Central Coast December – April June – September

Cultural Practices
There are many onion varieties planted in California. Onion varieties are generally classified by daylength
(short, intermediate, long), market use (fresh bulb, dehydrator bulb), and bulb color within the
fresh market class.
Dehydrator onions include short-day and long-day varieties. Short-day varieties (Creole, Creoso and
Primero) are selected for their high dry matter content with soluble solid contents ranging from 15 to
25%. Long-day dehydrators (Southport White Globe varieties) have soluble solid contents ranging from
20 to 25%. (13)
Fresh market bulb onions have the greatest number of varieties. Short-day market bulbs are usually
Grono or Granex-Grono types and include yellow, red, and white varieties. Yellow varieties include
Henry’s special, Mr. Max, Rio Enrique, Linda Vista, Monsoon, Colossal, Chula Vista, and Rio Bravo.
Red varieties include Rio Raji, Red Grano and Rojo. White varieties include Contessa, Diamante, Early
Supreme and Rio Rondo. "Imperial Sweets" can also be considered short-day varieties. Imperial Sweet
varieties range from five to seven percent soluble solid content, 2 to 6 moles of pyruvic acid per Kg of
bulb, and a high sugar to pyruvic acid ratio. (14)
Intermediate-day fresh market bulbs include varieties such as Stockton Yellow Globe, Stockton Red
Globe, Cal Red, Fresno Red, Yega, Yula, San Joaquin, Early Harvest, Cardinal, Early Red Burger, Rio
Corona, Dakata, Rio Seco, Cimarron, Utopia and many others. The intermediate-day varieties range
from seven to nine percent soluble solid contents and 5 to 9 moles of pyruvic acid per Kg of bulb. (14)
Long–day fresh market bulb onions are Sweet Spanish and Fiesta types. Varieties include Fiesta,
Blanco, Duro, Maya, Copra, Cache, Cheyenne, Dakota, Zuni, Tamara, Norstar, Valiant, Armada, Fuego,
Tesoro, Vaquero and Condor. The long-day varieties range from eight to 12 percent soluble solids and
10 to 20 moles of pyruvic acid per Kg of bulb. (14)
Onions are biennial monocots that are cultivated as annuals. It is a cool-season crop that requires
temperatures of at least 55° F to emerge. Optimum leaf growth rates occur at temperatures of 68° F to
77° F. Between the temperatures of 45° F and 50° F bolting will occur. California varieties begin to bulb
at 12 to 15 hour day lengths. (13,14)
Most commercial acreage is direct seeded however transplanting is used in some fall-planted fields to
produce an early harvest. Onions are planted in multiple rows on raise-beds that are 40 to 42 inches
wide. When sprinkler irrigation is used, four to six rows are planted on the bed top at equidistant. When
drip irrigation is used, four to six rows are planted with a space in the middle of the bed for the irrigation

tape. Seeds are planted half an inch deep in finely tilled, moist soil. The number of onion seeds per
pound range from 100,000 to 130,000. Dehydrator and fresh market varieties are sown at 4 to 5 pounds
per acre. (6,13,14)
Onions are grown on a wide variety of soil types: sand, loam, clay or organic/peat. The soil must be
uniform and clod-free. After seeding, onions must not be allowed to dry out. The soil is kept wet
throughout the preemergence/post-plant stage and once the plants have emerged, frequent irrigation is
required. The optimal time to irrigate is after 25 percent of available moisture has been depleted from
the top two feet of the soil. An average onion crop uses 30 to 36 inches of water per growing season.
(13,14)
When onions reach maturity, their leaves drop. Irrigation management at the time of harvest is crucial.
Water use is high until maturity begins, when it decreases rapidly. The timing on the last irrigation
requires special consideration, since stopping too soon will reduce yield, and stopping too late can cause
splitting, delay maturity and induce disease. The optimal time for final irrigation is usually when ten to
50 percent of the tops have fallen. (13,14)
The market sometimes requires that onions be harvested at only 20 to 50 percent mature bulbs. Fresh
market bulbs are harvested by hand. Hand harvesting involves rolling tops, undercutting with a rod
weeder and immediately harvesting to avoid sunburn. Crews pull and hand clip bulbs into buckets.
Bulbs are then placed into burlap bags and left in the field for 3 days to 2 weeks to cure. Curing allows
the onions to dry and form a protective outer layer that covers the bulb. After curing, the bags are loaded
into bulk trucks for transport to packing sheds, loaded into bulk bins for storage or field-packed. Packing
involves sizing bulbs into several groups: Colossal (> 4″ diameters), Jumbo (3 to 4″ ), Medium (2 ¼ to
3″ ), Repacks (1 ½ to 2 ¼ ″ ) and Boilers (

Insect Pests
In California, onion thrips and onion maggot are the most significant insect pests of onions. Other insect
pests include western flower thrips, seed corn maggot, pea leafminer, armyworms, bulb mites, and wheat
curl mites. (6,12)
Insect resistance to organophosphate, carbamate and pyrethroid chemicals is suspected in California
although it has not been documented. Resistance has been documented in other states. If multiple
treatments are required then alternating insecticides with different modes of action is recommended.
(6,12)
Onion thrips (Thrips tabaci) and Western Flower Thrips (Frankliniella occidentalis)
Thrips thrive in hot, dry environments; conditions found in many areas of California. Mature onion
thrips are 0.05 inches long and western flower thrips are 0.06 inches long. Adults have two pairs of
wings with long hairs while nymphs are wingless. Because adult thrips fly, they are particularly
important since they can act as vectors for pathogens such as purple blotch, Alternaria porri. (12)
Many reproduction cycles can occur every year. Typically one cycle is completed in two to four weeks;
however at temperatures over 85° F the life cycle takes 10 to 11 days. Thrips produce progeny by both
sexual and asexual reproduction. When asexual reproduction occurs only female offspring are produced.
Each mature female can lay up to 80 eggs that are deposited in the plant tissue. (2)
Damage occurs from the rasping and sucking feeding behavior of the nymphs and adults. Thrips rasp the
leaf surface then suck the plant fluid. This causes distinctive silvery streak spots on the leaves. As a rule
thrips feed in protected areas near the bulb; however, if there is a high population feeding will occur on
the outer leaf surfaces. In severe infestations the whole field will have a silvery appearance. Thrips can
reduce onion yield and quality. (2,12)
Thrips can be monitored one of two ways: either the foliage only or the whole onion plant is sampled.
The latter is the most effective monitoring method since it reveals thrips in the onion bulb and well as
under the leaf folds. Five samples should be taken from four areas of the field. Thirty thrips per plant at
mid-season is a suggested threshold although this will vary. (12)
CULTURAL CONTROL- Plant thrips-resistant cultivars such as Grano or Sweet Spanish and if
possible, grow onions during the rainy season. Watering from overhead irrigation and rainfall will
provide some thrips suppression. Because thrips suck on plant fluids, regular watering can minimize

damage. Under dry, hot conditions allow two to three weeks between each planting. New plantings
should be made up wind of older plantings to minimize dispersal. Planting onions in proximity to other
crops such as carrots will also aid in thrips management. (2,17)
BIOLOGICAL CONTROL- Beneficial organisms can be used to control thrips in bulb onions. These
include pink lady beetles (Coleomegilla imaculata), green lacewing larvae, minute pirate bugs (Orius
tristaclor), predatory mites (Neoseiulus cucumeris), lady bugs (Hippodamia convergens), and an insectspecific
fungus (Beauveria bassiana). None of these control agents provides acceptable control of thrips,
therefore, research on the role of natural enemies in the onion system is clearly needed. (2,7,11)
CHEMICAL CONTROL- Thrips must be controlled during the early bulbing stage of onions,
otherwise populations will increase to the point that control later will be extremely difficult if not
impossible. Typically, treatments are applied at the first sign of thrips. Thorough coverage of
insecticides is necessary since thrips hide in protected areas of plants. Even though onions can tolerate
high populations near harvest, control measures may be necessary to reduce the annoyance to the harvest
crew. (12)
● Cypermethrin was applied to 26.8% of California’s bulb onion crop acreage in 1997. Onions
were treated with 2,895 lbs. a.i. in 740 applications. Applications are made by chemigation, air or
ground at rates of 0.08 to 0.1 lbs. a.i. per acre and not to exceed 0.5 lbs. a.i. per acre per season.
The label preharvest interval (PHI) is 7 days, but the actual PHI range is 30 to 120 days. A permit
from the county agricultural commissioner’s office is required for purchase and use.
Cypermethrin can also be used to control onion maggot adults, armyworms and leafminers.
(3,5,6,11,12,13,14)
● Methomyl was applied to 12.9% of California’s bulb onion crop acreage in 1997, when 9,602
lbs. a.i. were applied in 343 treatments. Applications are made at the rate of 0.5 lbs. a.i. per acre
and repeated as needed not to exceed 5.4 lbs. a.i. per acre or eight applications. Typically, 5 to 7
day application intervals are required to obtain good population control. The label PHI is 7 days,
but the actual PHI range is 30 to 120 days. To purchase or use this chemical a permit is required.
Methomyl can also be used to control beet armyworms in onions. (3,5,6,11,12,13,14)
● Permethrin was applied to 12.8% of the bulb onion crop acreage in California in 1997, when
1,999 lbs. a.i. were applied in 214 treatments. Applications by chemigation, air and ground are
made at rates of 0.15 to 0.3 lbs. a.i. per acre and can be repeated up to a maximum of 2 lbs. a.i.
per acre per season. Label PHI is 1 day, but the actual PHI range is 30 to 120 days. A permit is
required for purchase and use in California. Permethrin can also be used to control adult onion
maggots, armyworms and leafminers. (3,5,6,11,12,13,14)
● Diazinon was used to spray 8.02% of the bulb onion crops in California in 1997, when 11,287
lbs. a.i. were applied in 219 treatments. Aerial, ground and chemigation applications of diazinon
are made at the rate of 0.5 lbs. a.i. per acre and repeated after 7 days if necessary. Do not make

more than three applications per season. The label PHI is 14 days, but the actual PHI range is 30
to 120 days. A permit is required for purchase and use. (3,5,6,11,12,13,14)
● Malathion was applied to 4% of the California bulb onion crop in 1997 when 6,522 lbs. a.i. were
applied in 138 treatments. This chemical is applied by ground, air or chemigation at the rate of
0.94 lbs. a.i per acre. Malathion has a 3 day PHI, but the actual PHI range is 30 to 120 days. This
chemical can also be used to control onion maggots. (3,5,6,11,13,14)
● Methyl parathion was sprayed on 3.3% of the California bulb onion acreage in 1997, using
1,619 lbs. a.i. in 64 treatments. Ground or aerial applications can be applied at 0.5 lbs. a.i. per
acre and repeated as needed. Methyl parathion has a 15-day PHI. A permit is required for its
purchase and use in California. No use of methyl parathion has been reported for the Imperial
Valley. (3,5,6,11,12,13,14)
● Oxamyl was applied to 2.1% of California’s bulb onion acreage in 1997, when 1,179 lbs. a.i.
were applied in 52 treatments. Ground, aerial or chemigation applications can be made at rates of
0.5 to 1 lb. a.i. per acre and repeated at 14-day intervals as needed not to exceed 4.5 lbs. a.i. per
acre per growing season. Oxamyl has a PHI of 14 days. In California, use of this chemical is
limited to Modoc and Siskiyou Counties as required by the SLN (special local need) label. This
chemical is used most often to control nematodes. (3,5,11,13,14)
● Azinphos-methyl was applied to 0.02% of California’s bulb onion acreage in 1997, when 12 lbs.
a.i. were applied in 2 treatments. Applications can be made by ground or air at rates of 0.5 to 0.75
lbs. a.i. per acre. Subsequent applications can be made at 7-day intervals as needed. The PHI is
28 days. Some formulations list a one-day PHI on the label. (3,5,11,13,14)
● Azadirachtin was applied to 0.4% of the crop in 21 applications in California in 1997. A total of
3 lbs. a.i. was used. This chemical can be applied by ground or air at rates of 0.1 to 0.7 oz a.i. per
acre and repeated at seven day intervals. If the thrips population is high the applications can be
repeated at three to four day intervals and no more than 0.7 oz a.i. per acre can be applied per
season. Azadirachtin can be applied up until the day of harvest. This compound can also be used
to control onion maggots, armyworms and leafminers. (3,5,11,13,14)
● Insecticidal soap was applied to 0.2% of California’s bulb onions, in 9 treatments using 279 lbs.
a.i. in 1997. It can be applied at the first sign of thrips and repeated at seven to 14 day intervals.
Rates of 1 to 2% v/v solution can be used. The PHI is 14 days. This chemical can also be used as
a leafminer control agent. (5,11,13,14)
● Petroleum oil was not applied in 1997. This product can be applied, as needed by air or ground
at rates of 1 to 2 gallons per 100 gallons of water. No PHI has been established for petroleum oil.
It can also be used to control leafminers. (3,5,11,13,14)

● Pyrethrins were applied in 43 onion treatments in 1997, using a total of 6 lbs. a.i. Applications
can be made by ground or air at rates of 1 to 6 lbs. a.i. per acre as needed. No PHI has been
established for pyrethrins. It can also be used to control onion maggots and leafminers. (5,11)
● Rotenone was applied in 41 treatments in 1997, when 4 lbs. a.i. was used. Rates of 0.004 to
0.009 lbs. a.i. per acre are applied as needed to maintain control of thrips populations. Rotenone
may be tank-mixed with pyrethrins. No PHI has been established. Rotenone can also be used to
control leafminers and mites. (5,11,12)
● Lambda-cyhalothrin can be applied by chemigation, air or ground at rates of 0.02 to 0.03 lbs. a.
i. per acre for thrips control. Applications can be made at intervals of no less than 5 days, not to
exceed 0.24 lbs. a.i. per acre per season. The PHI is 14 days. Lambda-cyhalothrin can be used to
control armyworms and adult onion maggots, seed corn maggots, and leafminers. This chemical
has been registered recently, so use information is not yet available. (3,5)
● Zeta-cypermethrin can be applied at the first sign of thrips. Applications can be made by
chemigation, air or ground at the rates of 0.0375 to 0.05 lbs. a.i. per acre not to exceed 0.25 lbs. a.
i. per acre. The PHI is 7 days. Adult onion maggots, armyworms and leafminers are also
controlled by zeta-cypermethrin. This chemical has been registered recently so use information is
not yet available. (3,5)
Onion maggot (Delia antiqua) and Seed corn maggot (D. platura).
Seed corn maggot can be found in onion crops throughout California whereas, onion maggot is restricted
to the cooler coastal areas. These pests are 0.25-inches long, legless, whitish maggots found in most
soils high in organic matter. (12)
These insects are multivoltine. It takes approximately 45 to 65 days to complete one reproductive cycle.
The adult female lays clusters of approximately 200 eggs over a period of several days. These eggs are
deposited in the ground near the onion stalk. The eggs hatch after ten days. The newly emerged maggots
bore into the stem of the onion and feed on the bulb for two to three weeks, damaging the plants. The
maggots then drop to the soil to pupate until the adults emerge. (5,12,17)
The first sign of maggot damage may be stunted or wilted plants. Seedling plants are most susceptible to
severe damage. Seed corn maggots attack germinating seedlings whereas, onion maggots attack the plant
at any stage. (12,16)
No efficient method has been developed to monitor onion maggots, although, adult populations can be
monitored with yellow sticky traps. (12)

CULTURAL CONTROL- Avoid planting onions in fields high in organic matter. Volunteer onions
should be removed from fields rather than being plowed under. Avoid planting successive onion crops.
(12,16)
BIOLOGICAL CONTROL- Natural enemies of onion maggot and seed corn maggot include: lady
bugs, beetles, birds, parasitic wasps, nematodes, and parasitic fungi. Diatomaceous earth is another
nonchemical control that can be used. The use of sterile onion maggot flies to control populations is
under investigation. (5,17)
CHEMICAL CONTROL- The most effective insecticide application timing for control of onion
maggot and seed corn maggot is at planting. These treatments should be considered for fields with
histories of maggot problems and fields high in organic matter. If high fly adult populations becomes a
problem applications to control the adults may be desirable. (6,12)
● Chlorpyrifos was applied to 4.1% of California’s onion crop acreage in 1997, when 3,276 lbs. a.
i. were applied in 85 treatments. Applications can be made in-furrow at the time of planting to
kill larvae. A maximum of one treatment per season can be applied at rates of 0.9 to 1.05 lbs. a.i.
per acre. The actual PHI range is 200 to 240 days. (3,6,11,12,13,14)
● Cypermethrin is an onion maggot adulticide. See onion and western flower thrips.
● Diazinon – see onion and western flower thrips. Rates used to control onion maggot and seed
corn maggot larvae are 2 to 4 lbs. a.i. per acre. (5)
● Malathion – see onion and western flower thrips. Rates used to control onion maggot and seed
corn maggot larvae are 1.6 to 1.9 lbs. a.i. per acre. (5)
● Permethrin - see onion and western flower thrips. Rates used to control onion maggot and seed
corn maggot adults are 0.1 to 0.3 lbs. a.i. per acre. (5)
● Pyrethrins are used to treat adult onion maggots. See onion and western flower thrips.
● Azadirachtin is an onion maggot larvae treatment. See onion and western flower thrips.
● Lindane was applied 1 time in 1997 using 0.49 lb. a.i. to treat 1 acre of bulb onions. It can be
used at the rate of 0.81 fl. oz a.i. per 100 lbs. of seed to control seed corn maggots in onions.
(5,11,13,14)
Other chemicals that can be used to control onion maggots and seed corn maggots are listed below.
● Lambda-cyhalothrin - see onion and western flower thrips. Rates used to control onion maggot
and seed corn maggot adults are 0.015 to 0.025 lbs. a.i. per acre. (5)

● Zeta-cypermethrin is an onion maggot adulticide. See onion and western flower thrips.
● Cyromazine was given a Section 18 in California in 1998. This chemical can be used to control
seed corn and onion maggots. It is applied to seed at rates of 1 lb. per 15 lbs. seed. (4)
Pea leafminer (Liriomyza huidobrensis).
Pea leafminer is a black and yellow fly found in onion crops throughout California. Leafminer is not a
serious pest of onions unless populations become so high that foliage is prematurely killed. (12)
Leafminers produce numerous generations per year. Adult flies deposit white, oval eggs individually
inside foliage. After the eggs hatch, larvae go through four instars. It is during these larval stages that the
leafminer does its damage. After the last instar, the larvae drop and pupate in the ground. The life cycle
is complete when the adults emerge. (12)
Mines are damage markings made as leafminers feed on the inner tissue of leaves. The adult female also
makes stippling marks on the leaf surface when she feeds or deposits an egg. However adults cause
insignificant levels of damage to the foliage. (12)
Monitoring can be done by counting the number of mined leaves in randomly sampled plants throughout
the field. (12)
CULTURAL CONTROL- Planting onions in close proximity to other crops such as lettuce, celery, or
spinach that may have leafminer problems should be avoided. Minimize leafminer problems by working
soil well and allowing sufficient time for pupae in the soil to emerge from fields that have been
previously planted with lettuce, celery or spinach. (12)
BIOLOGICAL CONTROL- Natural enemies of onion maggot and seed corn maggot include: lady
bugs, beetles, birds, parasitic wasps, nematodes, and parasitic fungus. Diatomaceous earth is another
nonchemical control that can be used. The use of sterile onion maggot flies to control populations is
under investigation. (5,17)
CHEMICAL CONTROL- Management decisions should be based on the level of larval infestations in
the plants. (12)
● Cypermethrin - see onion and western thrips. Labeled rates used for adult leafminer control are
0.04 to 0.1 lbs. a.i per acre. (5)

● Permethrin - see onion and western flower thrips. Labeled rates used for adult leafminer control
are 0.1 to 0.3 lbs. a.i. per acre. (5)
● Azadirachtin - see onion and western flower thrips.
● Fatty acid salts - see onion and western flower thrips.
● Petroleum oil - see onion and western flower thrips.
● Pyrethins - see onion and western flower thrips.
● Rotenone - see onion and western flower thrips.
Other chemicals that can be used to control leafminers in onions are listed below.
● Lambda-cyhalothrin - see onion and western flower thrips. Rates used to control adult
leafminers are 0.015 to 0.025 lbs. a.i. per acre. (5)
● Zeta-cypermethrin - see onion and western flower thrips. Rates used for adult leafminers are
0.028 to 0.05 lbs. a.i. per acre. (5)
Beet armyworm (Spodoptera exigua).
Beet armyworm moths are found all year in California. Larvae are green, hairless with broad stripes
down each side. (9,12)
Beet armyworms produce many generations per year. Each reproduction cycle takes approximately 24 to
36 days. After mating, the female moth will lay up to 600 eggs in clusters of 80 on plant leaves. Damage
to onion plants occurs during the period 14 to 21 days after hatch, as armyworms complete their five
larval stages. After feeding on foliage, larvae pupate at a one-inch depth in the soil for 7 to 14 days until
they emerge as moths. (9,12,15)
Monitoring is done at regular intervals by counting larvae and noting damage on randomly sampled
plants through out the field. (12)
CULTURAL CONTROL- Fields should be disked immediately after harvest to destroy larvae and
pupae. Removing weeds along field borders will also help minimize armyworm problems. (12)
BIOLOGICAL CONTROL- Bacillus thuringiensis (B.t.) was applied in 4 treatments using 7 lbs. a.i.

in 1997. Rates typically used are 0.5 to 1.5 lbs. a.i. per acre. B.t. is not harmful to natural enemies and no
PHI has been established. (3,5,12)
Other beneficial organisms that can be used to control armyworms are Cotesia marginiventris,
Steinernema feltiae, S. carpocapsae and Trichogramma sp., Beauveria bassiana, diatomaceous earth,
and NPV. (5)
CHEMICAL CONTROL- Applications are made at the first sign of armyworms. (12)
● Cypermethrin - see onion and western flower thrips. Rates used to control armyworms are 0.04
to 0.1 lbs. a.i. per acre. (5)
● Methomyl - see onion and western flower thrips.
● Permethrin - see onion and western flower thrips. Rates for armyworm control are 0.15 to 0.3
lbs. a.i. per acre. (5)
● Azadirachtin - see onion and western flower thrips.
Other chemicals that are registered to control armyworms are listed below.
● Lambda-cyhalothrin - see onion and western flower thrips.
● Zeta-cypermethrin - see onion and western flower thrips. Rates used to control armyworms are
0.028 to 0.05 lbs. a.i. per acre. (5)
Bulb mite (Rhizoglyphus spp., Tyrophagus spp.).
Bulb mites are tiny pearl-like, legged insects that prefer cool and wet climates. Bulb mites can survive
on onion crop residues in the field until the crop residues are completely decomposed. (12)
Scattered patches of stunted plants are typical of fields infested with bulb mites. Damage occurs when
the mites penetrate outer layers of plant tissue. Mite damage also allows opportunistic pathogens to get
into the bulb. This results in bulb losses due to rot both in the field and in storage. (12)
No specific monitoring method has been developed for bulb mites except to examine onion bulbs under
the microscope. (12)
CULTURAL CONTROL- Flood irrigation and heavy rains inhibit bulb mite populations. Fallow fields

to allow complete decomposition of vegetation. Successive onion or garlic planting should be avoided.
Short crop rotation cycles will allow the mites to survive. (12)
BIOLOGICAL CONTROL- There is no biological control for the control of bulb mites in onions. (12)
CHEMICAL CONTROL- Metam sodium was applied in 59 treatments using 182,958 lbs. a.i. in 1997
on 2.5% of the bulb onion crop. It is injected into the soil at rates of 155 to 248 lbs. a.i. per acre and
must be applied at least 14-days before planting. No PHI is established. Metam sodium is primarily used
as a nematacide but is also used to control bulb mites. Use of metam sodium, however does not prevent
reinfestation through infested bulbs. A permit is required from the county agricultural commissioner for
purchase and use. Metam sodium can also be used to control nematodes, pinkroot, and weeds.
(11,12,13,14)
Wheat curl mite (Eriophyes tulipae).
Wheat curl mites are microscopic, white wormlike, organisms about 0.01 inch long and are more
elongated than bulb mites. (12)
Under favorable conditions, it takes eight to ten days for these mites to complete one reproductive cycle.
Female adults each lay three to 25 eggs. Eggs are laid one per day and are deposited on the foliage. Once
eggs hatch, the juveniles migrate to the bulb where they feed and reproduce, ultimately causing the bulb
to dry up. (7,12)
These mites can cause damage to onion bulbs in the field and in storage. Also, they can be a vector for
other rot-causing organisms. (7,12)
No in-field monitoring method for wheat curl mites has been developed. However, onion bulbs can be
examined under the microscope for mites. (12)
CULTURAL CONTROL- Flood irrigation and heavy rains can decrease populations. Avoid planting
in fields following onions, garlic, corn or sudangrass as the previous crop. Allow bulbs to dry at least
eight hours at 75° F. (12)
BIOLOGICAL CONTROL- There is no biological control method for wheat curl mites at the present
time. (12)
CHEMICAL CONTROL- There is no chemical control available for wheat curl mites at the present
time. (12)

Diseases
Diseases can pose a serious threat to California onion crops. Diseases include: bacterial soft rots, basal
rot, black mold, blue mold, botrytis leafspot, downy mildew, botrytis bulb rot, pinkroot, purple blotch,
stemphylium leafblight, sour skin, white rot and rust. No biological controls currently exist for onion
disease pathogens. (12)
Downy mildew (Peronospora destructor).
Downy mildew is the most important disease of dehydrator bulb and fresh market bulb onions. The
disease is caused by a fungus found throughout California, that survives on plant debris and can be
spread very rapidly though spores when conditions are optimal. These conditions are: temperatures of
43° F to 80° F and a minimum leaf wetness of 1.5 hours. (10,12,13,14)
Symptoms of downy mildew are grayish-white to purple colored growth on older leaves and yellowing
of leaf tissue that leads to a collapse of the onion tops. Large yellow, circular clumps of plants a few feet
in diameter may be scattered throughout the field. (12)
CULTURAL CONTROL- In fields where downy mildew has occurred, use a three-year rotation to
crops other than onions, garlic or leeks. Crops should be planted with disease-free seeds, sets or bulbs.
Onion fields should be well drained. Volunteer allium plants in, or around the field, should be destroyed.
There are a few red onion cultivars such as Calred that are resistant to downy mildew. (10, 12)
CHEMICAL CONTROL
● Metalaxyl was applied to 41.1% of California’s onion acreage in 1997, using 8,912 lbs. a.i. in
1,173 aerial, ground or chemigation treatments. When conditions are favorable for the disease but
before infestation, a rate of 1.5 lbs. a.i. per acre is applied and repeated at 14-day intervals not to
exceed four applications per crop. The label PHI is 7 days, but the actual PHI range is 30 to 120
days. Note that crops planted in fields for the following 12 months must be registered for
metalaxyl. It can be tank-mixed with chlorothalonil. Other onion diseases controlled by metalaxyl
are: purple blotch, stemiphylium leaf blight and botrytis leafspot. (3,5,6,11,12,13,14)
● Chlorothalonil was applied to 34.1% of California’s onion acreage in 1997 using 58,872 lbs. a.i.
in 964 treatments. Applications can be made by chemigation, air and ground when disease is first
noticed. Rates of 0.75 to 1.5 lbs. a.i. per acre can be applied every 7 to 10 days. The label PHI is
7 days but the actual PHI range is 30 to 120 days. Chlorothalonil can also be used to control

purple blotch, stemiphylium leaf blight, botrytis leafspot and pinkroot. (3,5,6,11,12,13,14)
● Mancozeb was applied to 31.8% of the California onion crop in 1997, when 70,240 lbs. a.i were
applied by chemigation, air and ground in 863 treatments. When the disease first appears, rates of
1.5 to 2.25 lbs. a.i. per acre can be applied and repeated at seven day intervals not to exceed 24
lbs. a.i. per acre per crop. The label PHI is 7 days, but the actual PHI range is 30 to 120 days.
Mancozeb can be used to control other onion diseases such as purple blotch, stemiphylium leaf
blight and botrytis leafspot. (3,5,6,11,12)
Several different copper compounds were applied to onions in 1997. Copper hydroxide was the
most frequently used compound. It was applied to 15.8% of the onion crops in 350 treatments
using 20,617 lbs. a.i. The remaining 1,120 lbs. a.i. consisted of various copper compounds
applied in 37 applications. Plants treated with copper should be four to six inches high.
Applications are made at 0.75 to 1.5 lbs. a.i. per acre and repeated at 7 to 10 day intervals. None
of the copper compounds have established PHI’s. (3,5,11,12,13,14)
● Iprodione was applied to 8.3% of the California onion crop in 1997, using 5,374 lbs. a.i. in 297
treatments. When conditions favor the disease, rates of 0.5 to 0.75 lbs. a.i. per acre can be applied
not to exceed five applications in a season. If iprodione is tank-mixed then ten treatments are
allowed. The label PHI is 7 days but the actual PHI range is 30 to 120 days. Iprodione can also be
used to control purple blotch and stemiphylium leaf blight. (3,5,6,11,12,13,14)
● Maneb was applied to 10.4% of the California onion crop in 1997, using 16,376 lbs. a.i. in 254
aerial and ground treatments. When the disease first appears, rates of 1.5 to 2.25 lbs. a.i. per acre
can be applied and repeated at seven day intervals as necessary not to exceed 24 lbs. a.i. per acre
per crop. Maneb can also be tank-mixed with fosetyl-aluminum. The PHI is 7 days. Maneb can
also be used to control purple blotch, stemphylium leaf blight and botrytis leafspot.
(3,5,6,11,12,13,14)
● Fosetyl-aluminum was applied to 2.3% of California’s onion acreage in 1997, when 4,847 lbs. a.
i. were applied in 136 ground and aerial treatments. When conditions favor downy mildew,
applications can be made at a rate of 4 lbs. a.i. per acre and repeated at seven day intervals not to
exceed seven applications per season. Fosetyl can be tank-mixed with Maneb. The PHI is 7 days.
This chemical can also be used to control purple blotch and stemiphylium leaf blight.
(3,5,6,11,12,13,14)
● Sulfur was used in 58 applications to onions in 1997. A total of 103,619 lbs. a.i. were applied by
air and ground to 5.2% of the acreage. Applications are made either when the disease first
appears or when the conditions are favorable for disease development. Rates of 2.4 to 9 lbs. a.i.
per acre can be used and repeated as needed. No PHI has been established for sulfur.
(3,5,6,11,12,13,14)

Purple blotch and Stemphylium leaf blight (Alternaria porri and Stemphylium vesicarium).
During the winter, stemphylium leaf blight and purple blotch are found throughout California. In the
spring, these diseases are usually only found in the Imperial Valley. Both diseases cause the same
symptoms and are treated the same way. Fungi that overwinter on plant debris in and around fields cause
these diseases. The fungi are favored by wet conditions. Infection is often associated with downy
mildew lesions. (10,12)
Symptoms of purple blotch and stemphylium leaf blight are water-soaked areas on leaves that gradually
develop into deep purple lesion and yellow streaks. The lesions extend along leaf margins in both
directions, causing leaves to die. Bulb decay may happen during and after harvest. The bulb neck
becomes soft and has a yellow to wine-red discoloration. The infection moves throughout the onion
tissues. The bulb dries out and takes on a papery texture towards the outside. (10,12)
CULTURAL CONTROL- The variety ‘Fiesta’ has tolerance to purple blotch and stemphylium leaf
blight, whereas Sweet Spanish onions are very susceptible to infection. Onions should be properly
handled and dried during harvesting. Infected onions should be destroyed. (12)
CHEMICAL CONTROL
● Metalaxyl - see downy mildew.
● Chlorothalonil – see downy mildew.
● Mancozeb – see downy mildew.
● Iprodione – see downy mildew.
● Maneb – see downy mildew.
● Fosetyl aluminum – see downy mildew.
● Vinclozolin was applied to 0.5% of onion crops in 1997, when 337 lbs. a.i. were applied in 15
ground and aerial treatments. Applications at planting can be made at the rate of 0.75 lbs. a.i. per
acre and repeated four to six weeks after crop emergence. No more than 10 lbs. a.i. per acre or
four treatments can be applied in one season. The PHI is 18 days. Vinclozolin can also be used to
control white rot. (3,5,6,11,13,14)

White rot (Sclertium cepivorum).
A fungus that can survive in soil as sclerotia and remain dormant up to 20 years without a host causes
White rot. White rot can occur in an onion crop with as little as one sclerotium per ten kilograms of soil.
The sclerotia are spread by irrigation water, equipment and plant material. Optimum temperatures for
white rot are from 60° F to 65° F. The disease does not occur at temperatures above 78° F. (12)
Symptoms of white rot will first appear on older leaves and will include yellowing of the leaves, leaf
dieback, wilting roots rotting, and fluffy white growth around the base of the bulb. Onion plants can
become infected at any stage, but the disease is most common during the second half of the growing
season. (12)
CULTURAL CONTROL- Proper equipment sanitation will prevent soil migrating from infected areas
to uninfected areas. Do not move cull bulbs, litter, and soil from infested to non-infested fields. Sets and
transplants can carry sclerotia, but onion seed only rarely carries sclerotia. Once a field becomes
infected, the field should not be used again for allium crops. (12)
CHEMICAL CONTROL
● Iprodione – see downy mildew.
● Metam sodium can also be used but does not provide reliable control. See bulb mites. (12)
● Methyl bromide is a fumigant that was applied in 5 treatments using 2,903 lbs. a.i. in 1997.
Typical rates are 180 to 300 lbs. per acre. Methyl bromide is often applied in a mixture with
chloropicrin. Basal rot is another onion disease for which methyl bromide is used. Methyl
bromide is being phased-out under the provisions of the Clean Air Act and will no longer be
available after 2005. (11,12,13,14)
● Vinclozolin – see purple blotch.
● Dicloran was not applied in 1997. In 1995, it was applied in three treatments using 66 lbs. a.i. It
can be applied by ground at the time of seeding. Typical rates of 15 to 24 lbs. a. i. per acre are
used. Dicloran is also used to treat botrytis bulb rot. (3,5,11)
● Thiophanate-methyl is applied to the seeds or directly in the furrows at planting at rates of 8.4
to 11.2 lbs. a.i. per acre. In 1997, it was applied in two treatments using 7 lbs. a.i. (5,11)
Botrytis leafspot (Botrytis cinerea).

A fungus causes botrytis leafspot. When leaves are wet for 20 or more hours and temperatures reach 75°
F, spores can become established. The spores produce enzymes that damage foliage by causing leaf
spots. When spotting becomes numerous, leaf tips dieback and entire leaves may be killed. (10,12)
CULTURAL CONTROL- Isolating seed fields away from fresh market and processing onion fields
can minimize leafspot since bulbs are a major source of spore inoculum. Fields should be on a three-year
rotation, away from onions, garlic or leeks during which time volunteer and cull onions are destroyed.
(10,12)
CHEMICAL CONTROL
● Chlorothalonil – see downy mildew.
● Mancozeb – see downy mildew.
● Maneb – see downy mildew.
● Iprodione – see downy mildew.
● Metalaxyl – see downy mildew.
Botrytis bulb rot (Botrytis allii).
A fungus that is found throughout California causes botrytis bulb rot. The fungus can survive on plant
residue and in the soil as sclerotinia for long periods of time. The sclerotinia germinate under moist,
warm conditions and produce airborne conidia that land on and infect onion tissue. The optimum
conditions for bulb rot are during periods of high relative humidity with temperatures ranging from 50°
F to 75° F. (12)
Symptoms of bulb rot normally appear during storage, however, the infection occurs in the field.
Symptoms are a softening of the bulb neck tissue, which becomes water-soaked and turns brown, and
later a gray felt-like growth appears on rotting scales. (12)
CULTURAL CONTROL- Minimizing damage to the bulbs during handling can reduce botrytis bulb
rot infection. Heavy or late applications of nitrogen fertilizer should be avoided. Harvest only when
crops are fully mature, and properly cure the bulbs before storage. Storing onions at temperatures of 41°
F or less with low relative humidity minimizes the disease incidence. (12)

CHEMICAL CONTROL
● Dicloran – see white rot.
Basal rot (Fusarium oxysporum f.sp. cepae).
Fungi that can survive indefinitely in the soil cause basal rot. Symptoms of basal rot are a progressive
yellowing and dieback from the leaf tip, dark brown roots and occasionally, a white fungal growth at the
base of the bulb. Basal rot is often associated with pink root, maggots, or other insect injury and is more
prevalent in transplants. Optimum conditions for infection are moist soils with temperatures ranging
from 79 to 82° F. Basal rot may progress after harvest if the bulbs are stored in moist conditions. (12)
CULTURAL CONTROL- Onions should not be planted in fields with a history of basal rot. Basal rot
infection can be minimized by a three to four year crop rotation (crops other than onions, garlic or
leeks), proper curing prior to storage and maintenance of cool storage temperatures. (12)
CHEMICAL CONTROL
● Methyl bromide – see white rot.
Pinkroot (Phoma terrestris).
A fungus that can survive in soil indefinitely causes pinkroot. Drainage water or dirty equipment can
easily spread the fungus. Optimum conditions for pinkroot are temperatures ranging from 75° F to 85° F
with high relative humidity. (12)
Pinkroot causes roots to turn pink, darken and eventually die. Roots produced thereafter become
diseased and die. This continues throughout the growing season, and results in stunted bulbs. (12)
CULTURAL CONTROL- Several varieties of onions such as Durango, Yula, Yellow Globe Danvers,
and others are resistant to certain strains of pinkroot. Crops should be on a five-year or more long-term
rotation out of onions and not be planted following a cereal crop. Proper fertilization, irrigation,
cultivation, and insect control will minimize infection. Solarization of the soil is also used in the San
Joaquin Valley to prevent pinkroot. (12)
CHEMICAL CONTROL

● Chloropicrin was applied in seven treatments using 4,069 lbs. a.i. in 1997. Applications by
injection are made two weeks prior to planting at rates of 200 to 400 lbs. per acre.
(3,5,11,12,13,14)
● Dichloropropene is a preplant treatment that is applied by ground or by injection at rates of 103
to 285 lbs. a.i. per acre. This pesticide can also be mixed with chloropicrin. It was applied in four
treatments using 10,801 lbs. a.i. in 1997. (3,5)
● Metam sodium - see bulb mites.
Bacterial soft rots (Erwinia carotovora carotovora, E. chrysantherni, Pseudomonas gladioli, and
Entererobacter cloacae).
Bacterial soft rots are caused by soil borne organisms that can be spread through irrigation or rainwater.
Symptoms appear at harvest or later. They can be characterized by softening and water soaking of one or
more of the inner fleshy scales of the bulb, yellowing of infected tissue and a softening of the bulb neck.
(12)
CULTURAL CONTROL- Once onions start to bulb, soft rot infections can be minimized by using
furrow irrigation rather than sprinkler irrigation. Harvest should be made only after onions have fully
matured and the onions should be cured prior to storage. (12)
CHEMICAL CONTROL- No chemical controls are available for soft rots. (12)
Black mold (Aspergillus niger).
Black mold is caused by a fungus that is found in desert areas of California. The fungus survives in the
soil on decaying organic matter and infects damaged onions by entering via wounds. Symptoms of black
mold are moldy growths appearing on the top or sides of bulbs, water soaked scales, and black masses of
spores that become visible between outer scales. (12)
CULTURAL CONTROL- There is no direct control of black mold, however infection can be reduced
if bruises and other damage to bulbs is minimized during handling, and if keeping storage and
transportation temperatures are maintained between 33° F and 55° F. (12)
CHEMICAL CONTROL- No chemical controls are available for black mold. (12)

Blue mold (Penicillium spp.).
Blue mold rot can be caused by several fungi. Infection occurs through wounds, bruises or uncured neck
tissue of the bulb and is favored at temperatures from 70° F to 77° F and high relative humidity.
Symptoms appear near of harvest time and during storage. Signs include water soaked areas on the outer
surface of the scales, a blue-green powdery mold on the surface of bulbs and finally,in advanced stages,
the bulbs disintegrate into a watery rot. (12)
CULTURAL CONTROL- Blue mold rot infection can be minimized by controlling insect damage,
proper curing and careful handling. Storing bulbs at temperatures of 41° F or less with low relative
humidity also helps minimize the impact of the disease. (12)
CHEMICAL CONTROL- No chemical controls are available for blue mold. (12)
Sour skin (Pseudomonas cepacia).
Sour skin is caused by the bacterium, Pseudomonas cepacia, an organism that can survive long periods
of time in the soil. It is splashed onto plants during rain or irrigation and enters the plant through wounds
caused by mechanical or insect damage. The optimum temperature for sour skin is 85° F. (12)
Symptoms of sour skin are wilting and dieback of infected leaves. The internal leaf tissue and the bulb
neck become soft, watery and tan-colored, slimy rings form throughout the bulb. (12)
CULTURAL CONTROL- Once the onions start to bulb, furrow irrigation should be used rather than
sprinkler irrigation. Sour skin can be minimized by allowing tops to fully mature prior to harvesting and
properly drying bulbs prior to storage. (12)
CHEMICAL CONTROL- No chemical control methods have been established for sour skin. (12)
Rust (Puccinia porri).
Rust is usually only a problem in onions in fields where the crop is planted next to a heavily infested
garlic crop. Symptoms are red to orange colored lesions on leaf blades. Heavily infested plants may turn
yellow and collapse prematurely. The size and quality of onion bulbs is severely reduced by rust
problems. (12)

CULTURAL CONTROL- Avoid planting in or near fields with a history of rust. (12)
CHEMICAL CONTROL- There are no chemical controls for rust in onions. (12)
Onion Smut (Urocystis cephalae).
Onion smut is caused by a fungus that can live saprophytically in the soil for many years. Symptoms
appear early in plant development when blisters are seen on the lower part of the stems near the ground.
After these blisters erupt a large number of black spores are released. Onion smut reduces both yield and
quality of onions. (10)
CULTURAL CONTROL- Avoid planting in fields with a history of onion smut. (10)
CHEMICAL CONTROL
● Thiram combined with carboxin was granted a Section 18 in California in 1998. Rates used are
6.0 fl. oz. per 100 seeds. In 1997, 528 lbs. thiram and 160 lbs. carboxin were used to treat seeds.
(4,10,11)
Nematodes
Three main types of nematodes associated with California onions are stem and bulb nematode, root knot
nematode and stubby root nematode. Of these pests, stem and bulb nematode and the root knot
nematode are major pests in California. (12)
Stem and bulb nematode (Ditylenchus dipsaci). The stem and bulb nematode lives inside the plant.
Significant damage occurs when the nematode penetrates the onion and destroys the bulb as it feeds.
Plants infested with this nematode may have the following symptoms: distorted and bloated plant tissue
with a spongy appearance; stunted plants with shortened and thickened leaves often with brown or
yellowish spots; soft tissue develops at the neck of the bulb and moves downward; scales that become
pale gray, and under dry conditions, bulbs desiccate and split at the base. Opportunistic infections by
other organisms such as bacteria, fungi and onion maggots are common and can result in further decay.

(12)
Root knot nematode (Meliodogyne hapla, M. incognita, M.. javanica and M. chitwoodi). The root
knot nematode enters the onion as a second stage juvenile and remains there to feed until it reaches
maturity. Mature female nematodes produce an egg mass just outside the root or just below the surface
of the root. Damage can result in severe stunting of the plant and stand reduction. Infestations cause
galls on roots. M. hapla produces small galls that are difficult to see while other nematodes produce
larger, more visible galls. (12)
Stubby root nematode (Paratrucgidirys spp.). The stubby root nematode is a migrating ectoparasite
that feeds on but does not penetrate the root. This nematode lays individual eggs directly in the soil.
Populations increase during cool, damp weather. Symptoms of feeding are extremely short roots with
yellow to brownish cast. Damage can result in stunted plants and stand reduction. (12)
CULTURAL CONTROL- Sanitizing equipment and preventing the movement of infested soil to
uninfested areas can decrease nematode migration. In addition, well-drained soil will prevent nematode
problems. The root knot nematodes (M. incognita, M. javanica and M. arenaria) are inactive at
temperatures below 64° F. Thus planting onions when the soil temperatures are in this range can
minimize nematode damage. Growing nonhost crops such as carrots and lettuce for several years helps
decrease stem and bulb nematode populations. However, this is not feasible with root knot nematode and
stubby root nematode because of their wide host range. (12)
BIOLOGICAL CONTROL- Myrothecium verrucaria, botanical extracts, pathogenic fungus, and
diatomaceous earth are biologically derived nematacides that may be used in onion crops. (5)
CHEMICAL CONTROL
● Oxamyl – see onion thrips and western flower thrips.
● Pyrethrins – see onion thrips and western flower thrips.
● Metam sodium – see bulb mites.
Weeds
Weeds can be problematic to onion producers. Onions are slow growing plants with shallow roots
and narrow, upright foliage and are among the least competitive of all crops. Several flushes of

weeds can occur during one crop cycle. These factors make onions susceptible to weed
competition. Weed management is further complicated by the fact that onions are sown in high
plant densities that makes hand-weeding and cultivation more difficult. Monitoring weeds in fields
and keeping records is a key weed management strategy. Records should indicate weed species, air
temperatures and time of year. (12)
Region of
California
Low
Desert
High
Desert
San
Joaquin
Valley
Northern
Mountain
Valleys
Central
Coast
Weeds
annual bluegrass,
canarygrass, little
mallow, nettleleaf
goosefoot
mustards, Russian thistle,
yellow and purple
nutsedge
barnyardgrass,
cheeseweed, common
lambsquarter, common
purslane, dodder,
groundcherry, London
rocket, nightshades,
yellow and purple
nutsedge, pigweeds,
shepherdspurse,
sowthistle
Common lambsquarter,
Kochia nightshade,
pigweeds, Russina thistle,
annual bluegrass, burning
nettle, common
groundsel, common
lambsquarter, common
purslane, hairy
nightshade, little mallow,
pigweeds,
shepherdspurse.

Following is a list of weeds that pose particular problems in onion crops:
Yellow sweet clover and white sweet clover. These are difficult to control because none of the
registered products are effective against clover. The best solution is to avoid planting onions in
fields with high clover infestations. (12)
Yellow and purple nutsedge. These are perennial weeds that are particular problems in spring and
summer planted crops. They reproduce from underground tubers that can survive in the soil for
several years. Deep plowing with moldboard plows results in 95 to 98% control of nutsedge. (12)
Annual bluegrass. In the lower Colorado River desert, annual bluegrass can reach very high plant
populations in a field and become difficult to control. In this area, some fields are no longer being
used to grow onions because of annual bluegrass. One reason for this is the regular use of selective
grass herbicides (sethoxydim and fluazifop) that do not control annual bluegrass. Herbicides that
control annual bluegrass in the San Joaquin Valley and coastal valleys (bensulide on onions) do
not effectively control annual bluegrass in the low desert area. Clethodim does control annual
bluegrass. (12)
Dodder. This is a parasitic weed that is problematic in the San Joaquin Valley and coastal growing
areas. The only solution available is to avoid growing onions in any field with dodder. (12)
CULTURAL CONTROL- Control weeds before they become established in rotational crops and
never allow weeds to set seed. Plant onions in fields relatively free of weeds. Mechanically cultivate
bed shoulders and furrows. After harvest, fields should be clean cultivated or a green manure crop
should be planted. Before planting preirrigate to stimulate weed seed germination then shallow
cultivate or flame with a propane burner to kill them. Soil solarization can also help control
weeds. (12)
BIOLOGICAL CONTROL- No biological controls exist for weeds in onion crops.
CHEMICAL CONTROL- Since onions are planted in high densities, it is not possible to hand
weed or mechanically cultivate the bed tops. Therefore, weed control on bed tops is dependent
upon herbicides. These chemicals have three general categories: preplant, preemergence, and
postplant postemergence. See Appendix 2. (12)
● DCPA is a preemergence herbicide that was applied to 19.6% of California’s onion acreage
in 1997 when 89,993 lbs. a.i. were applied in 402 ground and chemigation treatments. Rates
used are 4.5 to 10.5 lbs. a.i. per acre. It controls a broad range of grasses and broadleaf
weeds. This product is no longer being manufactured, and when existing supplies have been

depleted will no longer be available. (3,6,11,18,13,14)
● Metam sodium is a fumigant that provides control of most onion weeds with the exception
of clovers, little mallow and yellow nutsedge. See bulb mites. (12)
● Glyphosate was applied to 5.1% of onion crops in 1997 when 4,264 lbs. a.i. were applied in
75 ground treatments. Rates used are 1 to 4 lbs. a.i. per acre. Glyphosate is used
postemergence to the weeds but preemergence to the crop. It has a broad range of control
however it only provides partial control of problematic weeds such as yellow and purple
nutsedge, little mallow and clovers. Glyphosate has a three day preplant interval, and the
actual PHI range is 200 to 240 days. (3,6,11,12,13,14,18)
● Paraquat was applied to 0.9% of the onion crops in 1997 when 740 lbs. a.i. were applied in 9
ground treatments. Paraquat is used postemergence to the weed but preemergence to the
crop. Paraquat controls some broadleaf weeds and grasses when applied at rates of 0.625-
0.94l lbs. a.i. per acre. The PHI is 60 days. A permit is required for purchase or use of this
product. (3,6,11,12,18)
● Bensulide is a preplant-incorporated herbicide applied at 4 to 6 lbs. a.i. per acre. Please
note, the higher rate may cause injury to crops. In 1997, it was applied to 0.5% of the
acreage using 1,366 lbs. a.i. in 12 treatments. Bensulide controls a limited number of
broadleaf weeds and some grasses including common purslane, pigweeds and annual
bluegrass. No PHI is specified on the label. (3,6,11,12,18)
● Oxyflurofen is a postemergence herbicide that was applied to 51% of California’s onion
crop in 1997 when 9,030 lbs. a.i. were applied in 1,133 ground treatments. Applications are
made when onion plants are in the two to four true leaf stage. Rates used are 0.12 to 0.25
lbs. a.i. per acre. Oxyflurofen is used to control broadleaf weeds particularly little mallow
and it has some residual soil activity after treatment. The label PHI is 45 days, but the
actual PHI range is 60 to 150 days. (3,6,11,12,13,14,18)
● Bromoxynil, a postemergence herbicide, was applied to 31.7% of onions in 1997 when
10,841 lbs. a.i. were applied in 579 treatments by ground at the rates of 0.25 to 0.375 lbs. a.i.
per acre. Plants are treated in the two to four true leaf stage. It is important to apply this
with at least 50 gallons of water per acre otherwise it will severely injure or kill the onion
plants as well as the weeds. Bromoxynil is very effective against broadleaf weeds
particularly mustard species. The actual PHI range is 120 to 180 days. (6,11,12,13,14,18)
● Fluazifop-p-butyl is a postemergence herbicide that was applied to 9.7% of California’s
onion acreage in 1997 when 2,096 lbs. a.i. were applied in 142 ground and chemigation
treatments at the rates of 0.094 to 0.375 lbs. a.i. per acre. It is effective against grasses
except annual bluegrass. The label PHI is 45 days, but the actual PHI range is 50 to 120

days. (3,6,11,12,13,14,18)
● Pendimethalin is a preemergence herbicide that was applied to 16.4% of the onion crop in
California in 1997 when 11,150 lbs. a.i. were applied in 206 ground treatments. Applications
are made at rates of 0.618 to 1.237 lbs. a.i. per acre when crop is in the two to six true leaf
stage. Pendimethalin is not effective against emerged weeds. Pendimethalin controls some
broadleaf weeds such as pigweeds and common lambsquarter as well as many annual
grasses such as barnyardgrass, and annual bluegrass. The label PHI is 60 days, but the
actual PHI is 100 to 150 days. (3,6,11,12,13,14,18)
● Clethodim is a postemergence herbicide that was applied to 7.3% of onions in 1997, when
1,061 lbs. a.i. were applied in 111 treatments at the rates of 0.1 to 0.25 lbs. a.i. per acre. It is
effective against annual grasses and some perennial grasses. Specifically, it can control
annual bluegrass in the two- to three-leaf stage. The label PHI is 45 days, but the actual
PHI range is 50 to 120 days. (6,11,12,13,14)
● Sethoxydim is a postemergence herbicide that was applied to 8.2% of the California onion
acreage in 1997 when 1,541 lbs. a.i. was applied in 127 treatments by air or ground at rates
of 0.09 to 0.26 lbs. a.i. per acre. Sethoxydim is effective against many annual (except annual
bluegrass) and perennial grasses and is safe to the crop. The label PHI is 30 days, but the
actual PHI is 50 to 120 days. (3,6,11,12,13,14,18)
● Trifluralin is a preemergence herbicide that was applied to 0.9% of the onion crop, using
342 lbs. a.i. in 18 ground treatments. Rates used are 0.375 to 0.625 lbs. a.i. per acre. This
chemical is effective preemergence only and has no effect on emerged weeds. The label PHI
is 60 days. (11,13,14,18)
See Appendix 2 for relative ratings of herbicides.
AREAS OF RESEARCH
● Steve Koike at UCCE Monterey County will be screening new fungicides for control of
downy mildew on onions.
● Mike Davis at UC, Davis will be evaluating pesticides products and management techniques
for control of soft rot and white rot.
● Carl Bell at UCCE Imperial County, Steve Fennimore and Tom Lanini, UCD Vegetable
Crops and Milt McGiffen, UCR will be conducting field efficacy trials to find a replacement
for DCPA (Dacthal) as a preemergence herbicide.

Mike Davis
Plant Pathologist
Dept. of Plant Pathology
University of California Davis
(530) 752-0303
Carl Bell
Farm Advisor
University of California
Cooperative Extension
Imperial County
(760) 352-9474
Robert Ehn
Agricultural Consultant
1508 Tollhouse Rd.
Suite D
Clovis, CA
(559) 297-9322
Richard Smith
Farm Advisor
University of California
Cooperative Extension
Monterey County
(831) 759-7350
Kurt Hembree
Farm Advisor
University of California
Cooperative Extension
Fresno County
(559) 456-7556
Profile reviewed by
Contacts

California Pesticide Impact Assessment Program
(530) 754-8378
References
1. California Drug and Food Association, California production statistics 1996. Webpages:
www.cdfa.ca.gov/kids/commodities/onions.html#1
2. Cornell University, Cornell International Institute for Food, Agriculture and Development,
Global crop pests, Thrips tabaci, 1995. Webpages: www.aruba.nysaes.cornall.edu/ent/
hortcrops/english/thrips.html
3. Crop protection reference, 15 th ed., C&P Press, New York, 1999.
4. Department of Pesticide Regulation, Current section 18 emergency exemptions, 1999.
Webpages: www.cdpr.ca.gov/cgi-bin/sec18/list.pl?stat=4
5. Disease and insect control guide, Meister Publishing Company, Willoughby, OH, 1998.
6. Ehn, Robert. American Dehydrated Onion and Garlic Association (ADOGA), Dry bulb
onion production information, 1997.
7. Institut National de la Recherche Agroonomique, Eriophyes tulipae Keifer, 1998.
Webpages: www.inra.fr/HYPPZ/RAVAGEUR/6eritul.htm
8. National Agricultural Statistics Service, CASS, Commodity statistics, Onions, 1998.
Webpages: www.nass.usda.gov/ca/bul/709veg.htm#Onions,All
9. North Carolina State University, Insect and related pests of vegetables. Webpages:
ipmwww.ncsu.edu/AG295/html/beet_armyworm.htm
10. Oregon State University, Onions for dehydration, 1998. Webpages: www.agroindia.org/
horttech/prod/oniondhy.html
11. California Department of Pesticide Regulation. Annual Pesticide Use Report 1997.

12. University of California, Pest management guidelines, Pests of onions and garlic, 1999.
Webpages: www.ipm.ucdavis.edu/PMG/selectnewpest.onion-and-garlic.html
13. University of California, Vegetable Research and Information Center, Dehydrator onion
production in California, 1999. Webpages: vric.ucdavis.edu/richome/html/veginfo/
commodity/onion/oniondehy.pdf
14. University of California, Vegetable Research and Information Center, Fresh market onion
production in California, 1999. Webpages: vric.ucdavis.edu/richome/html/veginfo/
commodity/onion/onionfmkt.pdf
15. University of Florida Cooperative Extension, Institute of Food and Agricultural Sciences,
Armyworms, 1997. Webpages: www.edis.ifas.ufl.edu/scripts/htmlgen.exe?
DOCUMENT_in016
16. University of Maine Cooperative Extension, Pest management, Onion maggot Factsheet,
1994. Webpages: www.pmo.umext.maine.edu/factsh/onionm94.html
17. University of Vermont Extension, Onion pests, Entomology leaflet 76, 1997. Webpages:
www.ctr.uvm.edu/ctr/el/el76.html
18. Weed control manual, Meister Publishing Company, Willoughby, OH, 1998.
Appendices
Appendix 1. Major onion producing areas of California.
1. San Joaquin Valley
2. Low Desert
3. High Desert
4. Central Coast
5. Northern Mountain Valleys
S. A. Fennimore, N. L. Flewelling and S. J Richard
Extension Specialist, Post Graduate Researchers
UC Davis, Department of Vegetable Crops

1636 E. Alisal St.
Salinas, CA 93905
Appendix 2. Herbicide Effectiveness in Onion Weeds in California*
ANNUAL WEEDS MET BRO FLU OXY PAR SET GLY PEN CLE BEN
barley, hare C N C P P C C P - P
bluegrass, annual C N N P P N C C C C
canarygrass C N C P P C C C C C
cereals C N C P P C C P - N
chickweed, common C N N N C N C C N P
clovers N N N P P N P N N N
crabgrasses C N C N C C C C - C
cudweeds C C N N N N C N N N
docks C C N P P N C P N N
dodders C N N N C N C N N N
fiddlenecks C C N C P N C C N N
filarees C P N C P N P N N N
fleabane, hairy C C N P C N C N N N
foxtails C N C N C C C C - C
goosefoot C C N C C N C P N N
groundcherries C C N C C N C P N N
groundsel, common C C N C C N C N N N
henbit C C N C C N C C N N
horseweed C C N P P N C N N N
knotweed, common C P N P P N C C N C
lambsquarters, common C C N C P N C C N P

lettuce, prickly C C N C P N C N N N
lovegrasses C N C C P C C C - C
mallow, little N P N C N N P P N N
morningglories P C N C P N C N N N
mustards C C N C C N C N N N
nettles C C N C P N N N N N
nightshade, black P C N C C N C N N N
nightshade, hairy C C N C C N C N N N
oat, wild C N C P P C C P - N
panicum, fall C N C N P C C C - C
pigweeds C C N C C N C C N C
puncturevine C C N C C N C P N N
purslane, common C N N C C N C C N C
radish, wild C C N P C N C N N N
rocket, London C C N C C N C C N N
ryegrasses C N C N P C C C - P
shepherdspurse C C N C P N C N N N
sowthistles C C N C P N C N N N
sunflower, wild C C N C P N C N N N
thistle, Russian C C N P C N C P N N
PERENNIAL WEEDS
bermudagrass C N P N N P C N P C
bermudagrass
(seedling)
C N C N P C C C C N
bindweed, field C N N N N N P N N N
bindweed
(seedling)
C N N N P N C P N N

johnsongrass C N C N N C C N - C
johnsongrass
(seedling)
nutsedge,
purple
nutsedge,
yellow
* Taken from
UC IPM Pest
Management
Guidelines:
Onions/Garlic
webpages
Ratings
C = control
P = partial
control N =
no control
- = no
information
Herbicides
BEN =
bensulide
MET =
metam
sodium
BRO =
bromoxynil
OXY =
oxyfluorfen
C N C N C C C C - N
N N N N N N P N N N
P N N N N N P N N N

CLE =
clethodim
PAR =
paraquat
FLU =
fluazifop-Pbutyl
PEN =
pendimethalin
GLY =
glyphosate
SET =
sethoxydim
Database and web development by the NSF Center for Integrated Pest Managment located at North Carolina State
University. All materials may be used freely with credit to the USDA.